Hi, I am a new AGoT player. I just red the rule book and played alone as a demo play. Questions. In accordance with some contents in the rule book and the texts in some of cards, cards affiliated with a different house cost 2 additional gold to play to put on one's play area. How can a player have a chance to get other family's card into his/her hands? Each houses begins the game with their own family deck, right?

And, for the Lannister plot card <Hear me Roar>, it says 'House Lannister only', then, does the card take an effect to only the Lannister, or means that only the Lannister player can use this card?(For the latter case, it may go back to my first question again, are there any chances for other players to take someone else's card(from other family) even it's plot card? Sorry, it's my first demo play, so I know nothing (like, You know nothing, John Snow...) I am waiting for your voice~

It's because you can build your own decks (though 1 Core Set alone is not enough for that), You can include cards from any and all Houses, provided they don't have the text "House X only" (where X is Stark, Lannister, Baratheon, Greyjoy, Targaryen or Martell). To be allowed these cards in your deck, your House must match the X.

Note that plot cards, like events, are actually neutral, and most of them can be used in any deck (the exceptions are those with "House X only" - there are 2 in the Core Set, Hear Me Roar for Lannister and Mad King's Legacy for Targaryen).

Many of the rules in the rulebook don't apply to the out-of-box decks from the Core Set. They are there for when you customize or build your own deck. The "2 gold out-of-House" penalty for playing cards with an affiliation that does not match your House card is one of those rules.

The "House Lannister only" text on Hear Me Roar!, and similar "House X only" text on other cards, means nothing during a game. It doesn't affect the way the game is played in any way, who can use the card, or how that card's effect is resolved when it is played. It only signifies that those cards are "loyal" to that House and can only be built into a deck under that particular House card.

Thank you so much, Khdzlin and ktom. So, it seems that the rule book of the Core set contains all of rules that we might require to play AGoT LCG with all of the current expansions and of the coming-released expansions. And, you both mentioned about customizing(building) one's own deck with different families, then, is that optional so I can just play with a family deck, or rather mandatory so I have to mix them up to experience real AGoT. Also, if the latter is more likely true, how can I customize my own deck with different families?

You don't have to mix houses. It's relatively rare, as you need a really good reason to include a card that is going to cost you much more than its retail price. There are only a few cards which are good enough to see play in other houses on any regular basis.

Bottom line, until you have more experience, it's probably just fine not to mix houses.